This invention relates broadly to a preassembled bolt and washer and more particularly to a fastening system for clamping and securing a plastic workpiece to a supporting workpiece.
With the increasing use of plastic and plastic-like material to replace steel and other sheet metal materials, a particular fastening problem has become apparent. Though plastic materials do provide certain weight; cost and corrosive resistant advantages over various sheet metal materials, the plastic does suffer from an inability to accept a load without deformation. The well-known phenomena of creeping or cold flow prevents a plastic material from being an element in a tightly clamped fastening joint. In most threaded fastening applications it is imperative that the threaded fastener not lose a predetermined tension or the fastener will become loosened and ineffective in the joint. Obviously, if a plastic material is an upper sheet to be secured to a lower structure under the compressive load of a threaded fastener, such as a bolt, the natural physical tendency of the plastic would be to flow under such load, thus releasing the desired tension in the joint.
Various spring load distributing washers have been suggested with little success in solving the problem.
It is therefore a primary object of the invention to produce a fastening system which incorporates a threaded bolt and washer-like device which effectively eliminates the compressive load on a plastic workpiece.
A further object of the invention is to produce a preassembled fastening unit which is useful in fastening a plastic workpiece to a supporting structure.
Yet a further object of the invention is a preassembled fastening unit which minimizes the deliterious effects of a clamping load on a plastic workpiece while maximizing the size of a fastener and load-bearing capability of such fasteners.
The objects and advantages of the invention are obtained by the fastener unit described herein which incorporates an elongated threaded fastener, such as a bolt, preassembled with a washer-like device having a tubular sleeve portion and a radially extending flange spring portion at its upper extremity. The threaded fastener is provided with an annular protuberance of a predetermined radial dimension greater than the crest diameter of the thread located closely adjacent to the bearing surface of the head of the bolt. The washer-like device is provided with a radially inwardly extending protuberance at the uppermost extremity of the sleeve wall so that the washer and bolt may be snappingly preassembled to one another and yet be free spinning. The threaded shank will extend upwardly within the lower portion of the sleeve to insure that there is no interference between the shank and tapped mating hole in the support structure which could prevent the sleeve from receiving the clamping load of the fastened joint. The bolt may further be provided with an unthreaded shank region of a limited axial extent between the bearing surface of the head and the annular protuberance with the transverse dimension of the unthreaded shank region being approximately equal to the crest diameter of the shank thereby maximizing the loading capabilities of the bolt.
The spring portion of the washer is preferably configured to be substantially conical with the outermost perimeter being spaced upwardly from the lowermost extremity of the sleeve a distance substantially equal to the thickness of the plastic workpiece to be fastened. Spring characteristics of the radial flange may be varied by various alternative structures to reduce the spring load on the plastic while maximizing the total clamping load capacity of the column or load bearing strut created by the sleeve portion.
These and other objects will become apparent from the following detailed description taken in conjunction with the accompanying drawing.